1. Field of the Invention
The present invention relates to fuel injection devices for mixing fuel and compressed air and, more particularly, to fuel injection devices for gas turbine engines that include a conical swirler to impart a swirling motion to dispensed fuel for improved fuel atomization and combustion and to methods of manufacturing such devices.
2. Background of the Related Art
Most fuel injectors, for example, most fuel injectors for gas turbine engines, enhance fuel atomization during engine ignition and combustion sequences using kinetic energy of a flowing air or gas stream to shatter a fuel sheet into fine droplets, which are then introduced into a combustion chamber. Atomization of fuel is desirable because atomized fuel combusts more quickly, more completely, and more cleanly. Some fuel injectors employ air assist atomizers to deliver high pressure, high velocity air from an external source, which is then mixed with fuel. An example of an air assist fuel nozzle is disclosed in commonly assigned U.S. Pat. No. 6,688,534, the teachings of which are incorporated herein by reference.
Typically, with air assist atomizers, fuel and externally supplied air that is delivered at high pressure and high velocity are mixed internally, i.e., within the nozzle, before the fuel-air mixture is discharged through a discharge orifice into a combustion chamber. In practice, it is desirable to maintain the air flow rate at a minimum, therefore, air assist atomizers are characterized by providing a relatively small quantity of very high velocity, high pressure air. One prevalent disadvantage of air assist atomizers, however, are undesirable backpressures within the nozzle that result from internal mixing in the nozzle.
An alternative to air assist atomizers are airblast atomizers, including for example, pre-filming type airblast atomizers and cross-flow type airblast atomizers. An example of a cross-flow type airblast atomizer is disclosed in commonly assigned U.S. Pat. No. 6,539,724, the teachings of which are incorporated herein by reference.
Whether the fuel injector is of an air assist or an airblast type, air swirlers are an essential component used in fuel injectors and combustor domes to produce a swirling flow in the primary combustion zone for sustaining and stabilizing the combustion process of the fuel over a wide range of operating conditions in gas turbine combustors. In a conventional combustor of a combustion chamber, the swirling flow is primarily established by a combined use of airflow entering through the combustor dome, fuel injector, and the dilution air holes on the liner walls of the combustor. The swirling flow creates a central recirculation zone that draws a portion of the hot combustion gases produced in the combustion chamber back toward the incoming cold fuel-air mixture to assist fuel vaporization and mixing. As the engine speed increases, the hot recirculation gas is capable of sustaining the combusting spray at a wide range of stochiometric ratios without blowing out.
However, due to the need to reduce pollutants and control emissions in general, advanced combustor design allocates a large portion of the combustor airflow through the fuel injectors and dome swirlers to lower the flame temperature in the primary combustion zone. This design approach has further enhanced the influence of air swirlers in determining the performance of gas turbine engines.
To achieve high performance and to reduce emission of pollutants, air swirlers not only enhance fuel/air mixing and flow stabilization, but they also assist fuel atomization and droplet dispersion. Depending on the application, the geometry of the air swirlers can vary significantly ranging from axial and radial turning vanes to the use of angled-holes and airfoil-shaped turning vanes. Each swirler design contains specific features and advantages to meet the requirements of various combustor designs and applications.
Most conventional fuel injectors and dome swirlers utilize either axial swirler turning vanes (FIGS. 1A and 1B) or radial swirler turning vanes (FIGS. 2A and 2B) to generate swirling flows. Referring to FIGS. 1A and 1B, there is shown an axial swirler 1 that comprises a plurality of turning vanes 2 that, typically, are cut by a milling machine in a straight or helical profile along the central axis 3 of the swirler 1. The turning vanes 2 are positioned at a radial locus and are equally spaced apart in the circumferential direction about the central axis 3 of the swirler body 4. The region between the turning vanes 2 and the inner/outer confining walls form the passages of the airflow, which is shown by an arrow. The primary feature of the axial swirler 1 is that the airflow within the passages is forced to circle around the central axis 3 of the swirler body 4 in a spiral manner. As airflow emerges out of the passages and the retaining walls, it expands radially outward at an acute angle with respect to the central axis 3 of the swirler body 4.
Referring now to FIGS. 2A and 2B, there is shown a typically radial swirler 5. The vane geometry of the radial swirler 5 differs from that of the axial swirler 1 shown in FIGS. 1A and 1B. Specifically, the respective bases or base portions of the turning vanes 6 of radial swirler 5 are arranged on a vertical plane that is normal to the central axis 7 of the swirler body 8. This configuration forces the airflow (as shown by the arrow) to move within the passages and retaining walls radially inward towards the central axis 7 of the swirler body 8. Using the radial swirler vanes 6, a deflecting flow or passage wall 9 is usually required to turn the airflow in the axial direction.
Others have disclosed alternative solutions. For example, U.S. Pat. No. 4,842,197 to Simon, et al. discloses a fuel injection apparatus and associated method for providing a highly atomized fuel flow using a swirl-induced recirculation flow in the combustion chamber. The Simon, et al. apparatus comprises three concentric air streams. The innermost and outermost air streams impart circumferential swirls in opposite directions. The central air stream is free of swirl, imparting a stream of air radially inward that is deflected in an axial direction. The innermost air stream and the central air stream atomize the fuel. The outermost air stream forms a stable recirculation region.
Additionally, U.S. Pat. No. 5,144,804 to Koblish, et al. discloses an airblast fuel nozzle to improve cold ignition. The Koblish, et al. fuel nozzle includes an inner air swirl system comprising air inlet slots spaced circumferentially about the nozzle body. Further, the air inlet slots include inner and outer tapered sections that provide an effective air swirl system.
Although both types of prior art air swirlers 1 and 5 demonstrate satisfactory results, axial swirlers 1 appear to be more widely used in fuel injectors. Axial swirlers 1 can be easily incorporated into common fuel injector devices, such as simplex airblast, pure airblast, and piloted airblast nozzles. They also are well suited for use in very small air passages to induce fluid swirl motion. The upstream opening of the turning vanes 2 in the axial swirler 1 is usually aligned with the incoming airflow, and, therefore, it does not encounter as much pressure loss from channeling the airflow into the vane passages as the radial swirler 5.
On the other hand, the radial swirlers 5 can be very effective in generating swirl flows without using aerodynamic turning vanes 6 with complex geometry. Radial swirlers 5 are largely used in the combustor dome. Using the simple straight vane geometry, a radial swirler 5 is capable of creating strong swirl and thorough mixing with little concern of the problem of aerodynamic wake flows.
A major disadvantage of axial and radial swirlers of the prior art is the means or method by which they are manufactured. Typically, axial and radial swirlers are manufactured by CNC milling machines. The machining process involves removing material one pass at a time along a certain trajectory or profile in a slow turning or profiling mode. This process is extremely time consuming. Further, when high-temperature hardened materials are required for the swirlers, the milling process becomes even more time-consuming and the tooling cost usually increases significantly due to more frequent changes of the cutting tools. To become competitive in today's world market, manufacturers must develop new machining techniques and swirler designs to reduce the manufacturing costs of fuel injectors and combustion domes.
Therefore, it would be desirable to provide a novel method of designing and fabricating air swirlers for use in fuel injectors and combustor domes. Furthermore, it would be desirable to provide novel swirler configurations that permit use of a more accurate and efficient technique to fabricate multiple parts simultaneously in multiple stacks, promising a significant reduction of manufacturing cost. Finally, it would be desirable to provide fuel injector designs that incorporate the concept of cone-shaped swirlers for improved fuel atomization and combustion performance.